It has been known for nearly 50 years that bone is piezoelectric, and that stress-strain of bone creates small voltage potentials which encourage and direct the deposition of various calcium salts. Interestingly, when bone is deformed within its elastic limits, the concave side becomes the cathode (electronegative) and all new bone deposition occurs here as opposed to the anode side. It is through this mechanism that a fractured bone heals.
Over the years investigators have claimed the value of intermittent mechanical loading in promoting and insuring fracture healing. Numerous devices have been developed to direct external voltage via implanted electrodes or by external stimulation using pulsed electromagnetic fields. Most of these applications typically employ externally applied voltage but frequently employ invasive procedures.
Systems that facilitate the healing of traumatized tissue and broken or fractured bone have been described, for example, by Boetzkes in U.S. Pat. Nos. 5,038,780 and 5,324,314. Such systems generally involve establishing an electric field between a pair of electrodes positioned on opposite sides of a patient site, resulting in the production of an alternating current having a desired frequency and amplitude characteristic in the tissue or bone. Such systems usually include a resonator formed by an inductor coupled in series with the resistor and capacitor of an equivalent circuit representing the patient site, the electrodes and any gaps therebetween. The resonator is run by an oscillator and also includes a capacitor.
Another method for promoting bone tissue growth and healing of bone tissue is described by McLeod et al. in U.S. Pat. No. 5,273,028. McLeod et al. provide an apparatus for applying a mechanical load to the bone tissue at a relatively low level on the order of between 50 and 500 microstrain peak-to-peak, and at a relatively high frequency in the range of about 15 and 55 Hz. The apparatus essentially consists of a rigid plate or area sufficient to support a patient's body when the spine is upright. The plate is supported by a stiffly compliant means and one or more transducers beneath the plate are provided to vertically drive the plate with referencing reaction to the relatively rigid support. The arrangement of a spring with the compliant means together with the plate-supported body mass is such that a naturally resonant frequency in the range of 10 Hz to 50 Hz is exhibited.
As described by M. Blank (Biosystems 35:175-178 (1995), medical studies have shown that application of low frequency electromagnetic fields accelerate the healing of bone fractures. Other studies such as one by Al-Holou (Biomedical Sciences Instrumentation 34:181-185 (1998)) confirm that induction of electric current in bone not only prevents the bone loss of functional disuse, but also induces new bone formation. As described by Al-Holou, the overall consensus in the medical community is that the skeletal response is optimal at a distinct frequency range of 10-30 Hz. It is thought that applications of very low strains may generate an effective osteogenic stimulation.
Research has demonstrated that the resonant frequency of a partially healed bone is considerably lower than that of a completely intact bone. As the healing of a fractured bone progresses, the resonant frequency rises until at full healing it matches the frequency of the opposite uninjured bone. Studies, such as those by Tower et al. (J. Orthop. Trauma 7:552-557 (1993)), demonstrated that there is a correlation between traditional parameters of tibial fracture healing and the measured resonant frequency of the healing tibia.
What is needed therefore, is a method of determining optimal frequencies of mechanical stimulation of bones, bone tissue, and bone fractures to obtain maximum healing. In addition, what is needed is a method for adjusting frequency according to the level of healing and for accommodating maximum healing through resonant frequency stimulation.